1. Field of the Invention
The present invention relates to improvements in non-aqueous electrolyte batteries, such as lithium-ion batteries and polymer batteries, and in methods of manufacturing the batteries. More particularly, the invention relates to a non-aqueous electrolyte battery that has excellent safety during overcharge and a method of manufacturing the battery.
2. Description of Related Art
Rapid advancements in size and weight reduction of mobile information terminal devices such as mobile telephones, notebook computers, and PDAs in recent years have created demand for higher capacity batteries as driving power sources for the devices. With their high energy density and high capacity, non-aqueous electrolyte batteries that perform charge and discharge by transferring lithium ions between the positive and negative electrodes have been widely used as the driving power sources for the mobile information terminal devices. Moreover, utilizing their characteristics, applications of non-aqueous electrolyte batteries, especially Li-ion batteries, have recently been broadened to middle-sized and large-sized batteries for power tools, electric automobiles, hybrid automobiles, etc., as well as mobile applications such as mobile telephones. As a consequence, demand for increased battery safety has been on the rise, along with demand for increased capacity and higher output power.
Many of commercially available non-aqueous electrolyte batteries, especially Li-ion batteries, adopt lithium cobalt oxide as their positive electrode active material. The energy that can be attained by lithium cobalt oxide, however, has almost reached the limit already. Therefore, to achieve higher battery capacity, it has been inevitable to increase the filling density of the positive electrode active material. Nevertheless, increasing the filling density of the positive electrode active material causes battery safety to degrade when the battery is overcharged. In other words, since there is a trade-off between improvement in battery capacity and enhancement in battery safety, improvements in capacity of the battery have lately made little progress. Even if a new positive electrode active material that can serve as an alternative to lithium cobalt oxide will be developed in the future, the necessity of increasing the filling density of the positive electrode active material to achieve a further higher capacity will still remain the same because the energy that can be attained by that newly developed active material will also reach the limit sooner or later.
Conventional unit cells incorporate various safety mechanisms such as a separator shutdown function and additives to electrolyte solutions, but these mechanisms are designed assuming a condition in which the filling density of active material is not very high. For that reason, increasing the filling density of active material as described above brings about such problems as follows. Since the infiltrating performance of the electrolyte solution into the interior of the electrodes is greatly reduced, reactions occur locally, causing lithium to deposit on the negative electrode surface. In addition, the convection of electrolyte solution is worsened and heat is entrapped within the electrodes, worsening heat dissipation. These prevent the above-mentioned safety mechanisms from fully exhibiting their functions, leading to further degradation in safety. Thus, it is necessary to establish a battery design that can make full use of those safety mechanisms without considerably compromising conventional battery designs.
To resolve the foregoing problems, various techniques have been proposed. For example, Japanese Published Unexamined Patent Application No. 2001-143705 proposes a Li-ion secondary battery that has improved safety using a positive electrode active material in which lithium cobalt oxide and lithium manganese oxide are mixed. Japanese Published Unexamined Patent Application No. 2001-143708 proposes a Li-ion secondary battery that improves storage performance and safety using a positive electrode active material in which two layers of lithium-nickel-cobalt composite oxides having different compositions are formed. Japanese Published Unexamined Patent Application No. 2001-338639 proposes a Li-ion secondary battery in which, for the purpose of enhancing battery safety determined by a nail penetration test, a plurality of layers are formed in the positive electrode and a material with high thermal stability is disposed in the lowermost layer of the positive electrode, to prevent the thermal runaway of the positive electrode due to heat that transfers via the current collector to the entire battery.
The above-described conventional batteries have the following problems.
(1) JP 2001-143705A
Merely mixing lithium cobalt oxide and lithium manganese oxide cannot fully exploit the advantage of lithium manganese oxide, which has excellent safety. Therefore, an improvement in safety cannot be attained.
(2) JP 2001-143708A
Lithium-nickel-cobalt composite oxide has lithium ions that can be abundantly extracted from its structure during overcharge. Since the lithium can deposit on the negative electrode and become a source of heat generation, it is difficult to improve the safety during overcharge and the like sufficiently.
(3) JP 2001-338639A
The above-described construction is intended for merely preventing the thermal runaway of a battery due to heat dissipation through the current collector under a certain voltage, and is not effective in preventing the thermal runaway of an active material that originates from deposited lithium on the negative electrode such as when overcharged. (The details will be discussed later.)